The invention relates generally to the field of electronic acoustic detection, and more particularly, to passive detection of boats by acoustic analysis and logic circuitry to discriminate between true and false signals.
The need for passive military intelligence about enemy activity has encouraged the development of various "listening" systems which monitor the level of vibrations in specified areas. Although the art of detecting motorized equipment on land has now reached a sophisticated level, detection of rivercraft, such as sampans, has remained such an elusive task that visual or photographic detection was relied on almost exclusively. In Southeast Asia the complex network of rivers and streams which are navigable by shallow draft boats has made it impossible for military personnel to maintain thorough surveillance of logistics routes. Simple acoustic detectors for combustion engine noise are only marginally effective on land because of omnipresent aircraft noise; but motorless sampans, rowed, sculled or poled are currently used anyway. Active sonar detection has been consistently ruled out due to a variety of false alarm producing characteristics of the rivers such as floating debris.
The most baffling detection problem is presented by the sculled sampan. This type of rivercraft, indigenous to Southeast Asia, is a long shallow draft barge with a single long oar extending aft. As in the Venetian gondola, the operator alternately shoves and pulls the pivoted oar in a transverse direction imparting a zigzag or tacking forward motion to the vessel. A skilled sculler generates practically no audible noise since the single oar is always in the water. Because of this fact, air-acoustic or hydroacoustic detection of sculled sampans, although desirable, appeared to be impractical.